The excitatory amino acid transporter 3 (EAAT3), also referred to in human studies as solute carrier family 1, member 1 (systematic gene name: SLC1A1) and in rodents as excitatory amino acid carrier 1 (EAAC1), is a high-affinity anionic amino acid transporter found in neurons throughout the cortex and in the hippocampus, basal ganglia (striatum, thalamus), and the olfactory bulb. EAAT3 functions to buffer local glutamate concentrations at excitatory synapses, for example in the hippocampus, and modulates the differential recruitment of glutamate receptor subtypes at extrasynaptic sites. Furthermore, EAAT3 is thought to be involved in facilitating GABA and glutathione biosynthesis. EAAT3 is a member of the EAAT family that mediates the uptake of glutamate into neuronal and glial cells of the mammalian CNS. Two transporters expressed primarily in glia, EAAT1 and EAAT2, are crucial for glutamate homeostasis in the adult mammalian brain and for rapid clearance of glutamate from the synaptic cleft. Three neuronal transporters (EAAT3, EAAT4, and EAAT5) appear to have additional functions in regulating and processing cellular excitability with EAAT3 being abundantly expressed throughout the CNS (EAAT4 is unique to Purkinje cells of the cerebellum and EAAT5 is expressed in rod photoreceptor and bipolar cells of the retina).EAATs are assembled as trimers, and the existence of multiple isoforms raises the question of whether certain isoforms can form hetero-oligomers. In the mammalian brain, the specificity of excitatory synaptic transmission depends on rapid diffusion of glutamate away from active synapses and the powerful uptake capacity of glutamate transporters in astrocytes. The extent to which neuronal glutamate transporters influence the lifetime of glutamate in the extracellular space remains unclear, but it is thought to be minor. EAAT3, the predominant neuronal glutamate transporter at excitatory synapses in hippocampal area CA1, buffers glutamate released during synaptic events and prolongs the time course of its clearance by astrocytes. EAAT3 does not significantly alter activation of receptors in the synaptic cleft. Instead, it reduces recruitment of perisynaptic/extrasynaptic NR2B-containing NMDARs, thereby facilitating induction of long-term potentiation by short bursts of high-frequency stimulation. Specific EAAT3 inhibitors may have the potential to locally and specifically strengthen particular synapses.
Obsessive-compulsive disorder (OCD) is among the most common mental disorders (prevalence 1-3%), and is at least as prevalent as schizophrenia and bipolar disorder. In the United States, one in 50 adults suffers from OCD. OCD affects children and adolescents as well as adults. Roughly one third to one half of adults with OCD reports a childhood onset of the disorder, and the disorder is typically chronic in nature. Treatment consists of predominantly serotonergic TCAs (clomipramine) or SSRIs in combination with cognitive-behavioral therapy (CBT). Overall, response to these interventions is of some but still limited benefit (approximately comparable to antidepressant response in MDD), and given the chronicity of OCD, the unmet medical need remains very high. OCD has been linked to serotonin and glutamate abnormalities. The hypothesis of glutamate signaling dysfunction in OCD is based on findings from neuroimaging, animal models, positional cloning and treatment studies.
The obsessive-compulsive symptomatology in OCD has considerable phenomenological, epidemiological and possibly (aetio)-pathophysiological overlap with a core autism spectrum disorder criterion: “restricted, repetitive patterns of behavior, interests, or activities” (taken from proposed DSM-5 revision). In support of this notion, human genetics studies have linked both the serotonin transporter and EAAT3 (SLC1A1) genes to autism spectrum disorder (ASD) or rigid-compulsive behavior in ASD and to OCD.
In addition, obsessive-compulsive symptoms induced by antipsychotics in schizophrenic bipolar disorder patients have been linked to EAAT3 (SLC1A1) gene variants. Post-mortem brain studies have shown that both classic and atypical antipsychotics reduce EAAT3, suggesting an involvement of this transporter in neuroleptic mechanisms beyond dopamine and serotonin modulation. Moreover, genetic variation in the human gene EAAT3 (SLC1A1) has been associated with antipsychotic drug response.
There is converging evidence from neurobiological data, human genetics, imaging studies and experimental treatments that EAAT3 is a key pathophysiological element in OCD and rigid-compulsive behavior in autism and in schizophrenia: A. A. Jensen et al., Curr. Opin. Pharmacol. 20, 116-123, 2015; L. A. Jarzylo abd H-Y. Man, J. Neurosci., 32, 2552-2563, 2012; A. Scimemi et al., J. Neurosci 29, 14581-14595, 2009; J. R. Wendland et al. Arch. Gen. Psychiatry, 66, 408-416, 2009; R. Bridges and C. S. Esslinger, Pharmacol. Ther. 107, 271-285, 2005; A. Nieoullon et al., J. Neurochem. 98, 1007-1018, 2006; K. Aoyama et al., Nat. Neurosci., 9, 119-126, 2006.